U.S. Pat. No. 5,138,880 discloses a digital level sensing probe system including digital probe constructed of two (2) concentric cylinders, forming a set of discrete, cylindrical capacitors that use the substance whose level is to be measured as a dielectric material. The set of capacitors is arranged along the axis of measurement where each capacitor represents a discrete level increment. Each capacitor is assigned a unique time slot in a switching sequence. The first level capacitor is used as a reference to which all other capacitors are sequentially compared. In the switching sequence, an AC signal is applied across each capacitor and compared with the inverted signal which is applied across the first level capacitor. A logic "1" results when the substance is present and a logic "0" when the substance is absent. The logic is decoded and converted to display the appropriate quantity of substance measured.
The possibility of mounting printed circuit configurations printed in the flat and mounted in facing relation on flat support elements with appropriate aligned spacing is mentioned in such Patent.
The use of individual isolated capacitors arranged along the axis of measurement, where each capacitor represents a discrete level increment checked sequentially in a unique time slot switching sequence for presence or absence of fluid, provides a marked advance in accuracy compared to analog probes which depend on variable dielectric values proportional to wetted level of the probe; also compared to semi-isolated capacitors employing a common electrode.